Field effect devices, such as field effect transistors, are fundamental components in modern electronics. They are basic components in most digital and many analog circuits, including circuits for data processing and telecommunications. Indeed it has been surmised that field effect transistors are among the most numerous of man-made objects.
Field effect devices typically comprise a conductive path between a source and a drain. A region of the path between the source and the drain, called the channel, is under the control of the electric field produced by a gate electrode. For example, the source and the drain can be n-type regions in the surface of a semiconductor substrate and the channel can be a shallow n-type region connecting them. A gate electrode formed on a thin insulator overlying the channel can be used to control the electrical properties of the channel. If no voltage is applied to the gate, current can flow from the source through the channel to the drain. However if a sufficient negative voltage is applied to the gate, electrons will be forced from the channel region, thereby depleting it of carriers and reducing or even shutting off the source-drain current. The highest operating frequency of such a device and, concomitantly its speed as a switch, is determined in large part by the shortness of the gate-channel region.
While reduction in gate length is one of the most effective ways to improve the high frequency performance and speed of field effect devices, further reduction in gate length has become increasingly difficult. The conventional approach to gate fabrication involves applying thin layers of insulator and conductor to a substrate and then defining the laterally extending gate structure by electron beam lithography. While electron beam lithography has, at considerable expense, reduced gate length as compared with the prior photolithographic processes, reproducible feature size is presently limited to dimensions of several hundred angstroms.